1. Field of the Invention
This invention relates to a surface acoustic wave substrate, more particularly to a surface acoustic wave substrate having a titanium nitride-based buffer layer sandwiched between a diamond layer and a piezoelectric layer.
2. Description of the Related Art
Since diamond exhibits a high acoustic velocity and thermal conductivity at room temperature, and since highly textured crystalline aluminum nitride (AlN) possesses piezoelectric characteristics, a high resistivity, a high breakdown voltage, and a high acoustic velocity, combination of aluminum nitride and diamond has been proposed for serving as a surface acoustic wave (SAW) substrate. Mortet, et al. (Appl. Phys. Lett., Vol. 81, (9), 1720, 2002), disclosed an approach in which a highly textured crystalline AlN film can be achieved by forming the AlN film on a smooth back side of a diamond film. Vogg, et al. (J. Appl. Phys, 96, (1), 895, 2004), disclosed another approach in which a highly textured AlN film can be achieved by forming the AlN film on a diamond film using Molecular Beam Epitaxy techniques. However, adhesion of the thus formed AlN film on the diamond film is relatively poor. As a consequence, delamination can easily occur for the AlN film, particularly when the layer thickness of the AlN film is greater than 2 μm.
U.S. Pat. No. 6,858,969 discloses a surface acoustic wave device (SAW device) that includes a Si substrate, a nanocrystalline diamond film formed on the Si substrate, metal electrodes formed on the diamond film, and piezoelectric composition coatings formed on the diamond film and the metal electrodes. The SAW device suffers the aforesaid delamination drawback for the metal electrodes and the piezoelectric composition coatings.